It is +that
It was my father who did the experiment in the lab yesterday evening.
It was Professor Wu that (who) sent me the letter.
It was I myself who opened the window.
It is this novel they talked about last week.
It was your mother-in-law that (whom) I met in the park the day before yesterday.
It was at an evening party that I first saw her.
It was when Hemingway lived in Cuba that he wrote The Old Man and the Sea.
It is only when one is ill that one realizes the importance of health.
It was not until recently that scientists know much about lung cancer.